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            <div><h4 class="title">
                <a name="asio.reference.async_read_at.overload2"></a><a class="link" href="overload2.html"
                                                                        title="async_read_at (2 of 4 overloads)">async_read_at
                (2 of 4 overloads)</a>
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    <p>
        Start an asynchronous operation to read a certain amount of data at the
        specified offset.
    </p>
    <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="../AsyncRandomAccessReadDevice.html"
                                             title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="../MutableBufferSequence.html"
                                             title="Mutable buffer sequence requirements">MutableBufferSequence</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="../CompletionCondition.html"
                                             title="Completion condition requirements">CompletionCondition</a><span
                class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="../ReadToken.html" title="Read token requirements">ReadToken</a> <span
                class="special">=</span> <a class="link"
                                            href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">&gt;</span>
<a class="link"
   href="../asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span
        class="emphasis"><em>DEDUCED</em></span></a> <span class="identifier">async_read_at</span><span class="special">(</span>
    <span class="identifier">AsyncRandomAccessReadDevice</span> <span class="special">&amp;</span> <span
                class="identifier">d</span><span class="special">,</span>
    <span class="identifier">uint64_t</span> <span class="identifier">offset</span><span class="special">,</span>
    <span class="keyword">const</span> <span class="identifier">MutableBufferSequence</span> <span
                class="special">&amp;</span> <span class="identifier">buffers</span><span class="special">,</span>
    <span class="identifier">CompletionCondition</span> <span class="identifier">completion_condition</span><span
                class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span
                class="identifier">token</span> <span class="special">=</span> <a class="link"
                                                                                  href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span
                class="emphasis"><em>DEFAULT</em></span></a><span class="special">);</span>
</pre>
    <p>
        This function is used to asynchronously read a certain number of bytes
        of data from a random access device at the specified offset. It is an initiating
        function for an <a class="link" href="../../overview/model/async_ops.html" title="Asynchronous Operations">asynchronous
        operation</a>, and always returns immediately. The asynchronous operation
        will continue until one of the following conditions is true:
    </p>
    <div class="itemizedlist">
        <ul class="itemizedlist" type="disc">
            <li class="listitem">
                The supplied buffers are full. That is, the bytes transferred is equal
                to the sum of the buffer sizes.
            </li>
            <li class="listitem">
                The completion_condition function object returns 0.
            </li>
        </ul>
    </div>
    <h6>
        <a name="asio.reference.async_read_at.overload2.h0"></a>
        <span><a name="asio.reference.async_read_at.overload2.parameters"></a></span><a class="link"
                                                                                        href="overload2.html#asio.reference.async_read_at.overload2.parameters">Parameters</a>
    </h6>
    <div class="variablelist">
        <p class="title"><b></b></p>
        <dl>
            <dt><span class="term">d</span></dt>
            <dd><p>
                The device from which the data is to be read. The type must support
                the AsyncRandomAccessReadDevice concept.
            </p></dd>
            <dt><span class="term">offset</span></dt>
            <dd><p>
                The offset at which the data will be read.
            </p></dd>
            <dt><span class="term">buffers</span></dt>
            <dd><p>
                One or more buffers into which the data will be read. The sum of
                the buffer sizes indicates the maximum number of bytes to read from
                the device. Although the buffers object may be copied as necessary,
                ownership of the underlying memory blocks is retained by the caller,
                which must guarantee that they remain valid until the completion
                handler is called.
            </p></dd>
            <dt><span class="term">completion_condition</span></dt>
            <dd>
                <p>
                    The function object to be called to determine whether the read operation
                    is complete. The signature of the function object must be:
                </p>
                <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span
                        class="identifier">size_t</span> <span class="identifier">completion_condition</span><span
                        class="special">(</span>
  <span class="comment">// Result of latest async_read_some_at operation.</span>
  <span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span
                            class="identifier">error_code</span><span class="special">&amp;</span> <span
                            class="identifier">error</span><span class="special">,</span>

  <span class="comment">// Number of bytes transferred so far.</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span
                            class="identifier">bytes_transferred</span>
<span class="special">);</span>
</pre>
                <p>
                    A return value of 0 indicates that the read operation is complete.
                    A non-zero return value indicates the maximum number of bytes to
                    be read on the next call to the device's async_read_some_at function.
                </p>
            </dd>
            <dt><span class="term">token</span></dt>
            <dd>
                <p>
                    The <a class="link" href="../../overview/model/completion_tokens.html" title="Completion Tokens">completion
                    token</a> that will be used to produce a completion handler, which
                    will be called when the read completes. Potential completion tokens
                    include <a class="link" href="../use_future.html" title="use_future"><code
                        class="computeroutput"><span class="identifier">use_future</span></code></a>,
                    <a class="link" href="../use_awaitable.html" title="use_awaitable"><code
                            class="computeroutput"><span class="identifier">use_awaitable</span></code></a>,
                    <a class="link" href="../yield_context.html" title="yield_context"><code
                            class="computeroutput"><span class="identifier">yield_context</span></code></a>,
                    or a function object with the correct completion signature. The function
                    signature of the completion handler must be:
                </p>
                <pre class="programlisting"><span class="keyword">void</span> <span
                        class="identifier">handler</span><span class="special">(</span>
  <span class="comment">// Result of operation.</span>
  <span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span
                            class="identifier">error_code</span><span class="special">&amp;</span> <span
                            class="identifier">error</span><span class="special">,</span>

  <span class="comment">// Number of bytes copied into the buffers. If an error</span>
  <span class="comment">// occurred, this will be the number of bytes successfully</span>
  <span class="comment">// transferred prior to the error.</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span
                            class="identifier">bytes_transferred</span>
<span class="special">);</span>
</pre>
                <p>
                    Regardless of whether the asynchronous operation completes immediately
                    or not, the completion handler will not be invoked from within this
                    function. On immediate completion, invocation of the handler will
                    be performed in a manner equivalent to using <a class="link" href="../post.html" title="post"><code
                        class="computeroutput"><span class="identifier">post</span></code></a>.
                </p>
            </dd>
        </dl>
    </div>
    <h6>
        <a name="asio.reference.async_read_at.overload2.h1"></a>
        <span><a name="asio.reference.async_read_at.overload2.completion_signature"></a></span><a class="link"
                                                                                                  href="overload2.html#asio.reference.async_read_at.overload2.completion_signature">Completion
        Signature</a>
    </h6>
    <pre class="programlisting"><span class="keyword">void</span><span class="special">(</span><span class="identifier">asio</span><span
            class="special">::</span><span class="identifier">error_code</span><span class="special">,</span> <span
            class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span
            class="special">)</span>
</pre>
    <h6>
        <a name="asio.reference.async_read_at.overload2.h2"></a>
        <span><a name="asio.reference.async_read_at.overload2.example"></a></span><a class="link"
                                                                                     href="overload2.html#asio.reference.async_read_at.overload2.example">Example</a>
    </h6>
    <p>
        To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code
            class="computeroutput"><span class="identifier">buffer</span></code></a> function as follows:
    </p>
    <pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">async_read_at</span><span class="special">(</span><span class="identifier">d</span><span
            class="special">,</span> <span class="number">42</span><span class="special">,</span>
    <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span
                class="special">(</span><span class="identifier">data</span><span class="special">,</span> <span
                class="identifier">size</span><span class="special">),</span>
    <span class="identifier">asio</span><span class="special">::</span><span class="identifier">transfer_at_least</span><span
                class="special">(</span><span class="number">32</span><span class="special">),</span>
    <span class="identifier">handler</span><span class="special">);</span>
</pre>
    <p>
        See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput"><span
            class="identifier">buffer</span></code></a>
        documentation for information on reading into multiple buffers in one go,
        and how to use it with arrays, boost::array or std::vector.
    </p>
    <h6>
        <a name="asio.reference.async_read_at.overload2.h3"></a>
        <span><a name="asio.reference.async_read_at.overload2.per_operation_cancellation"></a></span><a class="link"
                                                                                                        href="overload2.html#asio.reference.async_read_at.overload2.per_operation_cancellation">Per-Operation
        Cancellation</a>
    </h6>
    <p>
        This asynchronous operation supports cancellation for the following <a class="link"
                                                                               href="../cancellation_type.html"
                                                                               title="cancellation_type"><code
            class="computeroutput"><span class="identifier">cancellation_type</span></code></a>
        values:
    </p>
    <div class="itemizedlist">
        <ul class="itemizedlist" type="disc">
            <li class="listitem">
                <code class="computeroutput"><span class="identifier">cancellation_type</span><span
                        class="special">::</span><span class="identifier">terminal</span></code>
            </li>
            <li class="listitem">
                <code class="computeroutput"><span class="identifier">cancellation_type</span><span
                        class="special">::</span><span class="identifier">partial</span></code>
            </li>
        </ul>
    </div>
    <p>
        if they are also supported by the <code class="computeroutput"><span class="identifier">AsyncRandomAccessReadDevice</span></code>
        type's async_read_some_at operation.
    </p>
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                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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